This image shows the oscillating chemical wave on a rhodium nanoparticle. Image: TU Wien.
This image shows the oscillating chemical wave on a rhodium nanoparticle. Image: TU Wien.

Most commercial chemicals are produced using catalysts. Usually, these catalysts consist of tiny metal nanoparticles on an oxidic support. Similar to a cut diamond, which has a surface consisting of different facets oriented in different directions, a catalytic nanoparticle also possesses crystallographically different facets – and these facets can have different chemical properties.

Until now, these differences haven't often been considered in catalysis research, because it's very difficult to simultaneously obtain information about the chemical reaction and the surface structure of the catalyst. But this has now been achieved by researchers at the Vienna University of Technology (TU Wien) in Austria by combining different microscopic methods.

Utilizing field electron microscopy and field ion microscopy, the researchers were able to visualize the oxidation of hydrogen on a single rhodium nanoparticle in real time at nanometer resolution. This revealed surprising effects that will have to be taken into account in the search for better catalysts in the future. They report their findings in a paper in Science.

"In certain chemical reactions, a catalyst can periodically switch back and forth between an active and an inactive state," says Günter Rupprechter from the Institute of Materials Chemistry at TU Wien. "Self-sustaining chemical oscillations can occur between the two states – the chemist Gerhard Ertl received the Nobel Prize in Chemistry for this discovery in 2007."

These chemical oscillations happen on rhodium nanoparticles, which are used as a catalyst for hydrogen oxidation – the basis of every fuel cell. Under certain conditions, the rhodium nanoparticles can oscillate between a state in which oxygen molecules dissociate on the surface of the particle and a state in which hydrogen is bound.

"When a rhodium particle is exposed to an atmosphere of oxygen and hydrogen, the oxygen molecules are split into individual atoms at the rhodium surface," explains Yuri Suchorski, the first author of the paper. "These oxygen atoms can then migrate below the uppermost rhodium layer and accumulate as the subsurface oxygen there."

Through interaction with hydrogen, these stored oxygen atoms can then be brought out again to react with hydrogen atoms, which creates room for more oxygen atoms inside the rhodium particle and the cycle starts again. "This feedback mechanism controls the frequency of the oscillations", says Suchorski.

Until now, it was thought that these chemical oscillations always took place in the same rhythm over the entire nanoparticle. After all, the chemical processes on the different facets of the nanoparticle surface are spatially coupled, as the hydrogen atoms can easily migrate from one facet to the adjacent facets.

However, the results of the research groups of Rupprechter and Suchorski show that things are actually much more complex. Under certain conditions, the spatial coupling breaks and adjacent facets suddenly oscillate with significantly different frequencies – and in some regions of the nanoparticle, these oscillating 'chemical waves' do not propagate at all.

"This can be explained on an atomic scale," says Suchorski. "Under the influence of oxygen, protruding rows of rhodium atoms can emerge from a smooth surface." These rows of atoms can then act as a kind of 'wave breaker' and hamper the migration of hydrogen atoms from one facet to another – the facets become decoupled.

If this happens, the individual facets can form oscillations with different frequencies. "On different facets, the rhodium atoms are arranged differently on the surface," says Rupprechter. "That's why the incorporation of oxygen under the differing facets of the rhodium particle also proceeds at different rates, and so oscillations with different frequencies result on crystallographically different facets."

The key to unravelling this complex chemical behaviour lay in using a fine rhodium tip as a model for a catalytic nanoparticle. Applying an electric field to the tip caused electrons to leave due to the quantum mechanical tunnelling effect. These electrons are then accelerated by the electric field towards a screen, creating a projection image of the tip with a resolution of around 2nm.

In contrast to scanning microscopies, where the surface sites are scanned one after the other, such parallel imaging visualizes all surface atoms simultaneously – otherwise it would not be possible to monitor the synchronization and desynchronization of the oscillations.

These new insights into the interaction of individual facets of a nanoparticle can now lead to more effective catalysts, and provide deep atomic insights into mechanisms of non-linear reaction kinetics, pattern formation and spatial coupling.

This story is adapted from material from TU Wien, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.